The field of the invention pertains to the injection molding of plastic materials and, in particular, to means and methods of improving the strength of plastic to plastic weld lines or knit lines in molded products and improving the ability to pack out various areas of a molded cavity. The invention also addresses methodology for improved venting of molds and the improvement of gateblush in molded parts.
Often in the injection molding manufacture of plastic products multiple gates or multiple flow paths or both are required in the mold. Welds or knit lines occur in the plastic where the flow paths join. Because of certain mold or product designs and injection molding techniques a "cold" weld or knit line can occur because the plastic does not fuse together completely. The long chain polymeric molecules from each flow front at the knit line fail to fully re-entangle and regain the integrity of the solidified melt. The result is a very weak area in the plastic product. Many polymeric materials, especially filled materials and liquid crystal polymers, exhibit an extreme loss of knit line strength relative to the balance of the product. Thus, knit line strength failure is a major cause of production rejects and the structural failures of plastic products in service.
U.S. Pat. No. 2,191,703 discloses means and method for the elimination of knit lines and welds by accurately controlling the location of the knit lines and by providing cavities in communication with the mold cavity at the knit line locations. The continued application of injection pressure forces the newly formed knit lines out of the mold and into the cavities provided. The result is a product substantially devoid of weak knit lines. Unfortunately, after ejection from the mold the product must be trimmed from the plastic formed in the cavities and then the trim line polished if needed for appearance.
U.S. Pat. No. 2,199,144 discloses the use of tangential injection about a toroidal shape such as a steering wheel thereby creating a continuous flow about the mold and a smooth merger of flows at each tangential gate.
Both of the above disclosures are directed to toroidal shapes with toroidal shaped strengthening inserts. These approaches to solving the weld or knit line problem are only applicable to a limited number of geometries and not generally applicable.
U.S. Pat. No. 4,399,093 discloses the use of differing cooling rates in the two flows of the molten plastic resin that merge at the knit line. Initially the resin flows fill the mold at relatively low pressure. Upon filling the mold cavity a high packing pressure is applied causing the colder less viscous resin to protrude into the warmer more viscous resin at the knit line. The result is a knit line of greater surface area and interlocking shape to improve the strength at the knit line. However, the differing temperatures of the resin flows are dependent on product shape and cooling rates within the mold and, therefor, limited to product shapes and molds that can produce the differential cooling rates in the resin flows without affecting product quality.
With a view toward providing a method of injection molding that has wider applicability in improving the knit line strength of plastic injection molded products, applicant has developed the new method and apparatus disclosed below.